I have been told I need growth scans in pregnancy – why? Should I worry?

6 things to know:

1. Growth scans are carried out in women at higher risk of having a baby who might not grow as much as expected.
2. Scans are usually carried out at 2-4 week intervals.
3. The sonographer measures the baby’s head, thigh bone and tummy, and an estimated weight is calculated.
4. This estimated weight has subject to an error of about 10-15%, and tends to be more inaccurate in larger babies towards the end of pregnancy.
5. Both the actual estimated weight of the baby, and also how much the baby has grown since any previous scans are looked at.
6. Fluid around the baby and blood flows will also be measured. 

Why might I need growth scans?

You might need growth scans in pregnancy for lots of reasons. The most common reason is if you are at higher risk of having a baby who grows less than expected. Other reasons include if you had a previous small baby, if you have fibroids or if you are overweight, or if the midwife thinks your baby feeling a bit small or a bit large, or if you develop other concerns such as reduced fetal movements.  The first thing to say is try not to worry. Most of the time when scans are arranged to check baby’s size, it turns out that the baby is growing enough.

When would I have my growth scan?

Often, regular scans will be arranged every few weeks during pregnancy, usually between one scan every 2-4 weeks. Regular scans usually start around 28 to 32 weeks of pregnancy. The scans do not usually take too long, round 10-20 minutes depending on how cooperative your baby is!

What happens during a growth scan?

When you go for a scan, you do not need to have a full bladder. The sonographer will measure 3 main parts of your baby. This is like measuring a tape measure around the baby’s head (head circumference), around the baby’s tummy (abdominal circumference) and along the baby’s thigh bone (femur length). Often, the sonographer will measure these several times to make sure they are getting accurate measurements. Often babies move around a lot and practice breathing which can make getting accurate measurements challenging. These measurements are then put into a formula with calculates and estimated fetal weight for the baby. The measurements are plotted on a graph to compare the estimated size of the baby to the average, and also to compare how much the baby has grown compared to any previous scans. The sonographer will also measure the fluid around the baby and also the blood flow between the placenta and the baby. Sometimes other blood flows will be measured too. These measurements give an indication of who well the placenta is working.

Estimating a baby’s weight from ultrasound measurements is not an exact science. The formula for calculating an estimated fetal weight may not suit all babies and as baby’s can be difficult to measure, sometimes there are small errors in the measurements. We usually expect the estimated fetal weight to be accurate to within 10-15%. Scans tend to be more inaccurate towards the end of the pregnancy, or in women with a larger BMI. This 10% can make quite a big difference towards the end of the pregnancy, especially in larger babies.

Why are growth scans important?

Growth scans are carried out to try to identify babies who are not growing as much as they should due to the placenta not working so well. Many babies who measure small actually have normal placentas, but it is difficult to be certain which babies these are during pregnancy.